So in English when we say someone has "trouble focusing" it means they're having trouble paying attention. They're having trouble thinking about the thing they're supposed to be thinking about. The best example I can give for this is school. Maybe you're in school, or maybe you can remember school and think about that one teacher that was really boring. And when they were talking and you were supposed to be listening to them instead you were having "trouble focusing" and you were probably thinking about a whole bunch of other things that you weren't supposed to. So when you have "trouble focusing" it means that you're having trouble paying attention.